The present invention relates to an ultrasonic motor, and more particularly to an ultrasonic motor with improved rotation efficiency with respect to amount of power supplied thereto.
An ultrasonic motor includes a stator on which a piezoelectric element polarized in a plurality of magnetic poles is circumferentially disposed, and a disk-shaped or ring-shaped rotor rotatably butted to the stator at a predetermined pressure, and is configured to apply a high frequency voltage to the piezoelectric element of the stator to cause the piezoelectric element to vibrate, and to circumferentially expand the vibration with the comb teeth body integrally provided with the piezoelectric element for circumferentially moving the comb teeth body in a traveling wave form, thus to cause the rotor, which is frictionally engaged with the piezoelectric element, to rotate around the shaft.
FIG. 11 is a cross-sectional view showing an example of a conventional ultrasonic motor, which includes a short cylindrical container-shaped comb teeth body 12 with a plurality of comb teeth 121 circumferentially aligned thereon, integrally attached to a lower face of a disk-shaped base 11, and a flat disk-shaped piezoelectric element 13 circumferentially polarized in a plurality of magnetic poles respectively corresponding to the comb teeth 121, integrally mounted on an upper face of the comb teeth body 12, and the piezoelectric element 13 and the comb teeth body 12 constitute the stator 1. To the piezoelectric element 13, a high frequency voltage is to be applied via a flexible substrate 14. In a shaft hole 112 at the center of the base 11 a cylindrical sleeve 15 is fixed, and inside the sleeve 15 a ball bearing 17 is enclosed, so as to pivotally support a rotating shaft 3. To the rotating shaft 3, a short cylindrical shaped rotor 2 is attached, such that an upper end face of a peripheral wall portion 21 is butted to each of the comb teeth 121 of the comb teeth body 12. Also, a compressed coil spring 16 is inserted in an axial direction between a lower end portion of the sleeve 15 and the ball bearing 17, so that the axial elastic force of the compressed coil spring 16 biases the ball bearing 17 and the rotating shaft 3, thus to press the rotor 2 to the comb teeth 121 of the stator 1.
In this ultrasonic motor, when a high frequency voltage is applied to the piezoelectric element 13 via the flexible substrate 14, the piezoelectric element 13 vibrates, and hence the comb teeth body 12 integrally mounted therewith vibrates, to thereby circumferentially displace the comb teeth 121 which are circumferentially aligned. Accordingly, the peripheral wall portion 21 of the rotor 2 butted to the comb teeth 121 is equally moved circumferentially by the frictional force, so that the rotor 2 and the rotating shaft 3 integrally attached thereto are caused to rotate.
Thus, the rotor 2 is butted to the stator 1 at a predetermined pressure, and under such structure the rotor 2 has to be evenly pressed against the stator 1 in order to efficiently convert the vibration of the comb teeth body 12 of the stator 1 into the rotating motion of the rotor 1. For such purpose, the press-contact surfaces, via which the peripheral wall portion 21 of the rotor 2 and the comb teeth 12 of the stator 1 are butted with pressure, have to be processed at a precision of a level of several micrometers, and also the precision in assembly has to be upgraded so that the press-contact surfaces of the peripheral wall portion 21 and the comb teeth 121 may achieve a close contact. In an actual manufacturing process of the ultrasonic motor, however, it is practically difficult to satisfy such requirements, and hence a conversion loss into the rotating motion of the rotor 2 is inevitably incurred, thus resulting in failure in achieving an ultrasonic motor that provides higher rotation efficiency. It may be technically possible to upgrade the surface accuracy, however in this case the processing cost becomes significantly higher, which naturally leads to an increase in manufacturing cost of the ultrasonic motor.
Japanese Patent Provisional Publication No. P2002-58266A (hereinafter, referred to as '266 publication) adopts comb teeth of the stator that are elastically deformable in a rotation direction of the rotor. The technique according to '266 publication allows transmitting the vibration of the comb teeth to the rotor with the comb teeth closely pressed against the rotor because of the elastic deformation that takes place in the rotation direction of the rotor, which prevents a slipping motion between the stator and the rotor, thus effectively improving the rotation efficiency.
The technique according to '266 publication of elastically deforming the comb teeth of the stator in the rotation direction of the rotor is, however, not applicable to the ultrasonic motor as shown in FIG. 11, in which the comb teeth are circumferentially displaced to transmit the rotational force to the rotor. Even when the technique according to '266 publication is somehow applicable, the press-contact between the comb teeth and the rotor inevitably becomes uneven when the surface accuracy in a radial direction of the comb teeth and the rotor is insufficient, because the comb teeth are not designed to be elastically deformed radially. Consequently, the rotation efficiency cannot be improved as desired.